A switching power supply for converting an alternating current to a direct current is known. There is a device serving as the switching power supply which includes a burst mode (standby state) and is used in an image forming apparatus. The switching power supply executes full-wave rectification on an alternating current input with a diode bridge to generate a high-voltage direct current. The high-voltage direct current is input to a primary winding of a transformer. If a switching element is switched at a high frequency, a secondary winding of the transformer outputs a voltage. The output voltage of the secondary winding is rectified and smoothed by a diode and a capacitor to a low-voltage direct current. The output voltage of the secondary winding is used by a processor for controlling the whole of the image forming apparatus.
If the switching power supply is constituted as a flyback converter, there is an advantage in that the whole power conversion efficiency from the alternating current to the low-voltage direct current obtained by being rectified and smoothed becomes greater than or equal to 80%.
However, in the image forming apparatus, the time of a standby state is much longer than the time required in printing. Thus, a method of stopping power supply to a motor arranged in the image forming apparatus at the time of the standby state is employed.
On the other hand, as functions such as waiting for a print job, receiving a fax and the like from a network are enabled in advance, it is necessary that the electric power is supplied to the processor at the time of the standby state. Generally, a power supply requiring a large current has high efficiency at the time of the large current, and a power supply requiring a small current has high efficiency at the time of the small current. If a power supply for control used at the normal time and a power supply used in the standby state are arranged in the image forming apparatus and the two power supplies are switched in response to the state of the image forming apparatus, the cost becomes undesirably high.
In order to deal with a point that the power loss is large at the time of the relatively long standby state, a method of intermittently driving the switching element at the time of the standby state is adopted as a burst mode. Through this method, switching times can be reduced and the switching loss can also be reduced.
However, if the burst mode is used, a ripple of the output voltage of the secondary winding becomes undesirably large, and thus the burden of circuit design at the rear stage becomes too large and the cost is increased too high.